1. Field of the Invention
present invention generally relates to optical discs and, more particularly, is directed to an optical disc used as a record medium on which a variety of information signals can be recorded.
2. Description of the Prior Art
Conventionally, optical discs such as a magneto-optical disc and so on are known as recording media on which a variety of information signals can be recorded.
An optical disc using a rotating drive unit for rotating such an optical disc, one employing a magnetic clamp system for reducing a thickness of an information read and write apparatus is proposed. This disc rotating drive unit employing the magnetic clamp system is constructed such that a magnet is arranged on a disc table, and a magnetic metal plate, mounted on the optical disc, is magnetically attracted by this magnet to integrate the optical disc with the disc table for rotation.
Therefore, the optical disc to be mounted on the disc rotating drive unit employing the magnet clamp system comprises a hub made of synthetic resin deposited at a central portion of the disc body for holding the magnetic metal plate.
Conventionally, there is, as this kind of optical disc comprising a deposited hub, for example, "Japanese Utility Model Application No. 62-199240 (Japanese Utility Model Laid-open Gazette No. 1-107072), and so on.
A concept of this conventional optical disc is shown in FIGS. 1 to 3.
In the drawings, reference numeral 1 generally designates an optical disc, and this optical disc 1 is composed of a disc body 2 and a hub 5 having a metal plate 3 around the peripheral edge of which a resin ring 4 is integrally formed.
The disc body 2 is made by molding a synthetic resin, for example, polycarbonate resin, in a disc shape, wherein a single surface or both surfaces are used as recording surfaces on which recording tracks are formed in the circumferential direction, and further a circular central aperture 2a is bored at a central portion thereof.
On the other hand, the metal plate 3 comprised in the hub 5 is made of a magnetic material, for example, a steel plate or the like and formed in a disc shape, wherein a centering aperture 6, smaller than the central aperture 2a of the disc body 2 is bored at a central portion thereof. And, the resin ring 4 is formed integral with a peripheral edge portion of this metal plate 3 by an outsert molding method, to thereby complete the hub 5.
The surface of the resin ring 4 opposing the disc body 2 is provided with an annular rib 7 for welding the hub 5 on the disc body 2. Also, from an inner peripheral portion of the resin ring 4, a plurality of guiding ribs 8, which are slidably contacted with the inner peripheral surface of the central hole 2a of the disc body 2, are protrusively provided in a one-leg form.
Then, when this hub 5 is mounted on the disc body 2, the surface of the resin ring 4 on which the rib 7 for welding is formed is opposed to the surface of the disc body 2, the guiding rib 8 is inserted into the central hole 2a, and the hub 5 is positioned in a condition that the guiding ribs 8 are contacted with the inner peripheral surface of the central hole 2a. Then, an ultrasonic horn is pressed on the resin ring 4, wherein application of an ultrasonic vibration in a predetermined period of time causes the rib 7 to melt and weld to the hub 5 on the disc body 2.
Thus, the hub 5 is mounted on the disc body 2 in a condition where the centering hole 6 of the metal plate 3 is precisely coincident with the center of the track of the disc body 2, that is, a so-called aligning is achieved.
The optical disc thus constructed is generally accommodated in a disc cartridge for preservation. Upon loading, the metal plate 3 of the hub 5 is attracted by the magnet on the disc table dispose on the disc driving mechanism side, while the spindle shaft is inserted into the centering hole 6 for positioning, and the disc is driven to rotate in this clamped state.
The optical disc as mentioned above is generally left in a free condition in the disc cartridge, which many times results in the centering hole 6 of the hub 5 and the spindle shaft of the disc driving mechanism side not always being aligned on the same axial line upon loading. For this reason, the hub 5 is clamped by the magnet on the disc table in a manner that the centering hole 6 of the hub 5 is forcedly brought to the spindle shaft, which gives rise to a defect that the centering hole 6 and the spindle shaft are abraded.